1. Field of the Invention
The invention relates to an apparatus for and method of resecting corneal tissue, and more particularly to an apparatus for and method of resecting a thin corneal flap from the surface of the eye.
2. Description of the Related Art
Since the development of vision correcting lenses, ophthalmologists have continuously sought more permanent methods of correcting vision than glasses or contact lenses. Eye surgeons have tried, accepted, rejected, and modified various techniques in their quest for a simple, permanent procedure for correcting vision that retains its effectiveness over time and presents minimal side effects.
An early experimenter in the field was Jose I. Barraquer, who developed the technique of myopic keratomileusis. In this technique, the surgeon removes a lenticle, or corneal disk, from the corneal surface and affixes that corneal disk onto a cryolathe. The surgeon then reshapes the frozen disk, thaws the disk, and reattaches it to the stromal bed of the eye. Replacement of the reshaped disk results in a change of corneal curvature which in turn causes vision correction.
Barraquer also experimented with creating a flap on the surface of the cornea and then making a lamellar resection from the exposed stomal bed. The flap was then replaced, with the lamellar resection resulting in a flatter corneal curvature. This technique became known as in situ keratomileusis. The effectiveness of in situ keratomilcusis, however, was extremely sensitive to the accuracy of the lamellar resection taken from the stromal bed.
In both myopic keratomileusis using a cryolathe and in situ keratomileusis using a lamellar resection, the accuracy and depth of the cuts into the cornea of the eye were extremely important. Myopic keratomileusis required resection of a very uniform corneal disk, otherwise when the disk was turned on the cryolathe, the reshaping would cause irregularities in the ultimate vision correction. In situ keratomileusis was sensitive to the corneal cut for a different reason. The first cut had to be very even to form a uniform stromal surface from which to take the second resection. The second resection was even more sensitive to accuracy, because it was that resection that resulted in the vision correcting profile.
Because these techniques required such precise cuts, doctors and technicians developed a variety of extremely accurate cutting instruments known as microkeratomes. These instruments typically included a precisely aligned blade that was passed over the eye with its cutting edge perpendicular to the direction of travel. The blade was oscillated at high speed, preventing binding of the blade with the tissue being cut. The microkeratome was further modified and improved, for example, through the addition of micrometers and automatic advancement systems. This latter development, patented in U.S. Pat. No. 5,133,726 to Ruiz, et al., was necessary because the speed at which a microkeratome cuts the cornea was found to affect the thickness of the cut--again, of great importance in the early techniques.
While these techniques were maturing, a new laser device, the excimer laser, was making an impact in the field of eye surgery. This laser is a "cold" light laser, in that it breaks the molecular bonds through light rather than burning the tissue through heat. Because the excimer laser leaves surrounding tissue virtually unaffected, the excimer laser has become the preferred laser for operations on the cornea of the eye, and has been used in a number of techniques for removing tissue from the surface of the eye for correcting vision. An example for such a technique is found in PCT application PCT/EP93/02667 to Hohla.
The early Barraquer technique of in situ keratomileusis was greatly improved upon by Gholam Peyman, who married keratomileusis with the excimer laser. This technique is described in U.S. Pat. No. 4,840,175 to Peyman. The cornea is first resected, exposing the stromal tissue underneath. That exposed tissue is then ablated for refractive correction, and the corneal cap is replaced. This technique, known as laser in situ keratomileusis, has the advantage over "surface" based excimer laser techniques. These advantages in part flow from the structure of the eye itself. The cornea of the eye actually includes five layers, the outer three of which are illustrated in FIG. 1. The outer most layer is known is as the epithelium layer, denoted as layer 1 in FIG. 1, and is 50 to 90 microns thick. Bowman's membrane, denoted as layer 2, separates the epithelium from the substantiapropria, or stroma, layer 3. Bowman's membrane is about 12 microns thick. The stroma layer 3 makes up most of the thickness of the cornea, being from 400 to 450 microns thick.
Typical prior art excimer laser techniques first ablated away both the epithelium layer and Bowman's membrane before reaching the stroma layer. Peyman's technique instead provides for cutting into the stroma layer, then ablating the stroma layer, and then replacing that stroma layer. The end result is that neither the epithelium nor Bowman's membrane is affected. Peyman's technique eliminates trauma to the external surface layers of the cornea, resulting in improved healing and retained vision correction.
Peyman's technique has become known to ophthalmic surgeons as the "flap and zap" technique. To perform this technique, doctors typically use a microkeratome like those used for both myopic and in situ keratomileusis. While these microkeratomes, as noted above, are very precise instruments, they are also both expensive and unwieldy, and must be sterilized between uses. Further, they include high speed oscillating blades, as well as other complicated moving parts. It would be greatly desirable to simplify the resection of a corneal flap before performing the Peyman technique.